Optical waveguides have been known per se for a long time. These can, however, usually not be used in a high temperature environment, such as is present in the burner of a gas turbine system for example. The use of optical waveguides in burners of a gas turbine system is of particular interest in order to be able to observe the flame in the gas turbine system. For example, whether ignition has been successful, i.e. a flame has formed or the ignition has to be repeated, can then be optically determined during the ignition process. An optical monitoring of the flame is also useful during operation of the gas turbine system, for example, in order to check the stability of the flame.